This invention relates to a method of manufacturing various products by casting a slurry containing ceramics, metals, carbons, etc. in a mold and, in particular, to a method suitable for manufacturing products having an intricate configuration with a diversified wall thickness, such as a compressor scroll blade and a screw rotor.
Among the methods of molding various materials into products is slip casting, wherein the material powder is dispersed in a disperse medium (such as water or alcohol) to prepare a fluid slurry, which is poured into a mold to obtain a molded object. A lot of products are being manufactured by this molding method.
Usually, a gypsum mold is used in slip casting. However, when molding an object having an intricate configuration, such as a turbocharger rotor, a screw rotor, or a scroll blade, defects such as cracks are likely to be involved during the molding, so that, with a gypsum mold alone, it is difficult to mold such an intricate product. In view of this, the molding of a product with an intricate configuration by slip casting has conventionally been carried out by using, in combination, a gypsum mold and a mold which can be removed after the casting in the gypsum mold. The removable mold used may consist of a resin mold made of a thermoplastic or a thermosetting resin, a wax mold, or a rubber mold. Such a removable mold is integrated with the gypsum mold by adhesion, fitting, etc.
Molding methods of this type are disclosed, for example, in Japanese Patent Unexamined Publication Nos. 56-28687, 59-120405, 59-190811, 60-253505, 63-288703, etc.
In the method described in Japanese Unexamined Publication No. 63-288703, a polyethylene glycol, which is among polyalkylene glycols, is adopted as the material of the removable mold, which is melted and removed when releasing the molded object from the mold.
The properties of a polyethylene glycol, however, vary depending upon its molecular weight. For example, a low-molecular-weight polyethylene glycol has a molecular structure akin to that of alcohol and melts when absorbing water, etc., so that it cannot serve as the material of a core. On the other hand, a mold made of a high-molecular-weight polyethylene glycol exhibits flexibility in those sections thereof where it is in contact with the slurry. However, due to its large molecular weight, the flexibility resulting from its coming into contact with the slurry is far from satisfactory. Thus, when used as the material of a core, such a high-molecular-weight polyethylene glycol is not much different from a hard material except for those portions thereof constituting the core surface. Accordingly, it is not capable of absorbing the stresses generated when the mold absorbs dispersion medium to cause the molded object to shrink, with the result that cracks are generated in that process.
A problem in slip casting is that, if, when forming a green body (a molded object) by pouring slurry into a mold entirely consisting of gypsum, at least a part of the green body has a configuration which is liable to be restrained by the mold, the stresses that are generated as the green body shrinks cannot be mitigated, with the result that cracks are generated in the green body.
This is the same in the case where a resin mold and a gypsum mold are used in combination if the green body has any restrained portion, which will cause cracks to be generated therein when it is dried. Further, when removing the resin mold by heating, deformation of the molded object or generation of cracks therein may occur due to the thermal expansion of the resin.
This also applies to the case where a wax mold and a gypsum mold are used in combination. In this case, the crack generation and deformation are due to the poor flexibility of the wax mold or the gypsum mold. When using these two types of molds in combination, the operation of removing the removable mold by heating must be performed while maintaining a highly moist condition (which prevents the green body from drying). However, when decomposed by the high temperature when heating, the wax may soak into the green body, with the result that a large amount of carbons remains inside the green body. If calcining is carried out in that condition, the sintered body will be deformed, or the strength thereof is diminished.
Unlike the case where a resin mold or a wax mold is used, a combination of a rubber mold and a gypsum mold has an advantage that, due to the flexibility of rubber itself, crack generation may be avoided even if there exists a green body portion restrained by the mold. However, since the removal of the rubber mold is usually effected by burning it out at a temperature ranging from 450.degree. to 500.degree. C., this combination is not suitable for a case where the slurry contains a substance which is incompatible with oxidation, such as silicon carbide or silicon nitride. In addition, when removing the rubber mold by burning, the rubber mold may expand and deform, thereby causing crack generation and deformation in the molded object. Further, the heating temperature when removing the rubber mold is in excess of the boiling point of the dispersion medium, so that, when removing the rubber mold, it is necessary to dry the molded object to a sufficient degree so as to remove the dispersion medium therefrom, thereby avoiding generation of defects in the molded object due to boiling of the dispersion medium. However, such sound drying increases the shrinkage amount of the molded object, so that cracks may be generated due to the shrinkage of the molded object.
Further, in all the above-described cases, the mold is prepared by a very complicated method. The resin mold is prepared by injection molding using a metal mold, and the wax mold is prepared by the lost-wax process, wherein a model of the product to be obtained is prepared by injection molding using a water-soluble wax; the surface of this model is coated with a non-water-soluble wax, and the water-soluble wax is removed by dissolving it in water so as to obtain the wax model. When preparing the rubber mold, the material is subjected to maturing and hardening for a long period after being poured into a metal mold. Afterwards, the material is released from the metal mold.
All of these types of molds require a complicated preparation process, resulting in a high cost. It should also be noted that they are consumable goods.
Accordingly, there has been a request in slip casting that the mold for obtaining a product having an intricate configuration be prepared with ease, and that no cracks or deformation be generated in the molded object.